1. Field of the Invention
The invention relates to a method and apparatus for applying precoated heat activated adhesive tape to wood, and more particularly to a method and apparatus for activating a thermally activated adhesive tape by contacting the tape with the surface of a rim of a heated wheel.
2. Background Information
In the plywood industry manufacturing procedures often include adding a reinforcing tape to sheets of veneer before the veneer is laminated onto a sheet of plywood. This process takes place in a plywood composer machine in which sheets of veneer are placed edge to edge and tape is glued perpendicularly across the edges where the sheets touch. The veneer is generally a thin sheet of wood.
The term tape is used herewithin as a generic term referring to a flexible elongate reinforcing member comprising a tape, string, or cord. The cross section of the tape may be substantially rectangular, circular, or oval. The tape may comprise structural components such as strips, strands or filaments of various materials, including polymers such as polyester. The tape is generally impregnated with a heat activated hot melt adhesive. The hot melt adhesive is heated prior to being placed in contact with the wood and then cooled to cure the adhesive and form a strong bond between the tape and the wood.
In order to form a satisfactory bond the adhesive is raised to the correct application temperature prior to being placed in contact with the wood. Prior art equipment for applying thermally activated tape generally includes a heating element for softening and activating the adhesive and a cool shoe or wheel for pressing the heated tape onto the wood. In such equipment the heating portion generally includes a tube or slot through which the tape is drawn as it is heated. Typical application equipment is shown, for example, in U.S. Pat. No. 4,285,758 to Clausen et al., and U.S. Pat. Nos. 4,692,202 and 5,662,767 to Getz.
During application, as the adhesive-coated tape material is exposed to the heating element, the adhesive is heated to its softening point. Some of the adhesive may flow and separate from the tape before the tape is applied to the veneer. Thus, when using a conventional applicator, over a period of time the heating chamber accumulates the separated hot melt adhesive which tends to run through the heating tube and ultimately drip onto the veneer substrate. This dripped adhesive forms globs which are trapped between the sheets of veneer when the veneer is laminated to form plywood.
During the subsequent process of forming plywood, the glob of adhesive tends to be absorbed into the outer layers of veneer and through to the surface of the plywood. This forms unsightly blotches on the external surface. This problem is of particular concern in the preparation of hardwood plywood, which is used for cabinetry, and in other aesthetically sensitive applications. In such applications, the stain caused by a glob of glue seeping through the outer layer of veneer may substantially destroy the value of the sheet of plywood. The surface veneer of hardwood plywood tends to be particularly thin, and this thinness exacerbates the problem by increasing the probability that the excess glue will seep all the way through the veneer.
Also, the frictional and heat transfer characteristics of the heating tube found in conventional equipment limit the speed at which the tape may be applied. This, in turn, limits the production throughput of the composing equipment.
In addition, when the applicator is used to apply the tape material to the underside of a substrate, separated adhesive also drips onto the applicator frame or onto the floor as the heated tape material leaves the heating element. In this case, frequent cleaning of the dripped adhesive from the frame and floor is required to prevent debris from accumulating at the applicator, and production must often be curtailed for this routine maintenance.
Thus, a need exists for a method and an applicator for efficiently applying a tape with a thermally activated adhesive to the surface of wood veneer while overcoming the problems of prior applicators.
The present inventors have invented a method and an apparatus for efficiently applying a tape with a thermally activatable adhesive impregnated therein to the surface of a substrate, such as a wood veneer or decorative wood surfacing material, without the problems of the previous applicator.
According to one aspect of the present invention, a heated wheel with a low-adherence rim is provided. A heater such as an electric resistance heater in contact with the inside of the rim maintains the rim at an appropriate temperature. A tape which may include a reinforcing yarn, or filaments, and which is coated with a hot-melt adhesive, is supplied to the wheel adjacent the rim. The tape passes along a path that includes contact with the wheel over from about 2 to about 270 angular degrees or as much as about 315 angular degrees of the surface of the wheel rim. The wheel rim presses the tape to the wood as the wheel rolls across a broad surface of the substrate. The result is that, during the interval while the tape is in contact with the rim, the hot-melt adhesive with which it is impregnated is activated (melted), and when the tape is pressed to the surface of the wood, it adheres to the substrate. The tape is thus heated and transferred from the surface of the wheel to the surface of the wood, where it forms an adhesive reinforcing member unitizing one piece of substrate to another.
In the present invention, the interior portion of the wheel includes a polymer casting adjacent the heater. This casting provides structure to the wheel, and thermally insulates the heater from a bearing turning on a shaft at the center of the wheel.
An automatic controller, taking signals from a sensor near the rim of the wheel, can be used to maintain the rim at the correct temperature. The sensor may be embedded in the polymer casting. The signals from this sensor, as well as the power conducted by wires to the heater, pass through a slip-ring assembly mounted in a mounting arm that supports the wheel.
Thus, unwanted globs of glue within the plywood are avoided and the applicator apparatus and the floor around it remain free of waste adhesive.